Psycho-logical test.

My brother Matt sent me this:

This is an authentic psychological test:

It is a story about a girl--While at the funeral of her mother, she met a
man whom she did not know. She thought he was amazing, her dream guy, and
she fell in love with him but never asked for his number and could not find
him after the funeral. A few days later the girl killed her sister.

Question: What is her motive in killing her sister?

(To be continued in comments.)